Light olefins include ethylene, propylene and butene, and are important raw materials for synthesizing plastics, fibers and various chemical materials. With the growth of the national economy, the consumption of and demand for the light olefins are increasing rapidly. At present, the light olefins are produced mainly via the petrochemical route by naphtha cracking. With the decreasing global oil resources and the high price of crude oil, for countries especially like China with a shortage of oil resources, more than 60% of the consumed oil relies on imports every year. 70% of raw material for producing the light olefins comes from naphtha and light hydrocarbons obtained by refining crude oil in China. It is of important social significance and strategic significance to seek an alternative technological route from non-oil-based carbon resources such as coal, biomass and the like, for production of light olefins.
China has rich coal resources. Coal is used as the raw material and gasified to obtain synthesis gas (i.e., mixed gas of CO and H2); and the synthesis gas can be converted to methanol. The technology for production of olefins from methanol, i.e. methanol-to-olefins (MTO) is mature and is already industrialized. The catalyst is based on molecular sieves such as SAPO-34 or ZSM-5. The methanol conversion can reach close to 100%, and the selectivity of ethylene and propylene is 85% to 90%. Only a small amount of long-carbon-chain hydrocarbons (above C5+) are generated. In August 2010, the world's first commercial plant based on the DMTO technology developed by Dalian Institute of Chemical Physics (DICP) with independently intellectual property rights realizes commercial operation, representing a milestone progress in the emerging industries of producing olefins from coal in China. In April 2015, another 600k ton/year scale MTO plant was set up at Zhejiang Xingxing New Energy Co. Ltd., which also uses the core technology of DICP. It is the eighth set of large-scale industrial plant for production of olefins from coal-based methanol in China.
Compared with the indirect route of light olefins synthesis from coal-synthesis gas-methanol-light olefins, a direct synthesis route from synthesis gas by one-step process would make the process much simpler, much shorter process flow, much lower capital investment and production cost. To achieve direct conversion of synthesis gas to light olefins by one-step process, researchers from academia and industry have invested a great number of resources and efforts, studying on direct conversion of synthesis gas to light olefins. Sinopec Shanghai Research Institute of Petrochemical Technology, carried out economical assessment of the direct conversion technology based on the conventional Fischer-Tropsch technology from coal-based synthesis gas [Yang Xueping, Dong Li. Progress in Chemical Engineering 31 (2012) 1726-1731], named as FTTO technology in this patent. The FTTO technology had advantages in the selling cost when the oil price was $110/Barrel, coal price was not higher than RMB 520/ton, and the olefins concentration in all products was 30%. If the light olefins concentration in all products was higher than 40%, the economic advantage of the FTTO technology will become more advantageous.
Direct conversion of synthesis gas to light olefins can be achieved via the traditional Fischer-Tropsch synthesis route. In that process, metal and metal carbides are used as catalysts. CO is generally considered to adsorb and dissociate on the surface of the catalyst and is hydrogenated to generate CHx intermediate species on the surface. These surface CHx intermediates go through polymerization on the catalyst surface, growing into longer carbon chain hydrocarbons, thereby forming hydrocarbons with a wide range of carbon numbers. Therefore, the process involves a series of elemental reactions, such as C—O bond breaking, C—H bond formation (i.e. hydrogenation), and C—C coupling. Typical product distribution follows the Anderson-Schultz-Flory (ASF) distribution model, which can be expressed as ln(Wn/n)=nlnP+ln[(1−P)2/P], wherein Wn means the mass percent of hydrocarbon containing n carbon atoms, P means the chain growth probability, and (1−P) means the chain end probability. Polymerization degree D equals 1/(1−P). Once P is determined, the entire product distribution is determined. The characteristics of Fischer-Tropsch process is a wide distribution of hydrocarbons with different carbon atom numbers. For example, the selectivity of C2-C4 hydrocarbon is no more than 58%, while the highest selectivity of gasoline fraction (C5-C11) is 45%. In the meanwhile, a large amount of methane and higher alkane are generated. How to achieve selective synthesis of light olefins directly from synthesis gas remains a challenge since Fischer-Tropsch synthesis technology was invented. Researchers from all over the world have made a great deal of efforts, taking a variety of measures trying to improve the selectivity of light olefins, for example, by modifying the catalyst structures and compositions, in order to modify the reaction rates of different elemental steps in the process, such as methanation, hydrogenation, secondary reaction of light olefins, carbon chain growth and the like.
Fe-based catalysts have the advantages of low cost, easy availability, high activity, high selectivity of light olefins and the like, and were considered to be the most promising catalysts for synthesis of light olefin directly from synthesis gas. Researchers consequently have improved the selectivity of light olefins by adding additives with different components such as alkali metal K, Na and their salts, and transition metals such as Mn, Cu. Ruhr Chemical Corporation of Germany developed an iron-based catalyst promoted with multi-components Fe—Zn—Mn—K, which can catalyze direct conversion of synthesis gas to light olefins. Jingchang Zhang et al. from Beijing University of Chemical Technology reported a Fe—Mn—K/AC catalyst prepared using ferric oxalate as the precursor. That catalyst gave a CO conversion as high as 97% at a space velocity of 600 h−1, 15 bars and 320° C.; and the selectivity of C2=-C4= in hydrocarbons was 68% (excluding CO2) [Zhang Jingchang, Wei Guobin, Cao Weiliang, Chinese Journal of Catalysis 24(2003)259-264], which exceeded the selectivity limit of C2-C4 hydrocarbons predicted by the ASF distribution model.
The studies showed that the catalyst support materials also has a very important modification role in the product selectivity through interaction with Fe species. The research group of Professor de Jong reported a 12 wt % Fe catalyst supported on carbon nanofiber (CNF) and a-Al2O3 using ferric ammonium citrate as the precursor. Under reaction conditions of low pressure (1 bar), 350° C. and H2/CO=1, at a reaction time of 15 hours, the CO conversion was 0.5%-1.0% and the selectivity of light olefins among hydrocarbons was 60% [H. M. T. Galvis, J. H. Bitter, C. B. Hhare, M. Ruitenbeek, A. L. Dugulan, K. P. de Jong, Science 335 (2012) 835-838]. The same catalyst under conditions of 340° C., H2/CO=1, 20 bars and space velocity of 1500 h−1, a CO conversion of 70%-88% was obtained. The space time yield was 2.98×10−5 mol CO/gFe·s and 1.35×10−5 mol CO/gFe·s, respectively. The selectivity of CO2 was 42%-46%, and the selectivity of light olefins among all hydrocarbons was 52%-53%. Later, they reported that a small amount of 0.03% of S and about 0.2% of Na added into the catalyst improved obviously the activity and the selectivity of the light olefins [H. M. T. Galvis, A. C. J. Koeken, J. H. Bitter, T. Davidian, M. Ruitenbeek, A. I. Dugulan, K. P. de Jong, J. Catal. 303 (2013) 22-30]. The researchers from Dalian Institute of Chemical Physics of the Chinese Academy of Sciences studied systematically the effects of activated carbon as the support. They found that the products on the activated carbon supported iron catalysts deviated from the ASF distribution model [Shen Jianyi, Lin Liwu, Zhang Su, and Liang dongBai, Journal of Fuel Chemistry and Technology 19 (1991) 289-297; Ma Wenping, Ding Yunjie, Luo Hongyuan, et al., Chinese Journal of Catalysis, 22 (2001)279-282]. In addition, the catalyst preparation methods and conditions, such as the conditions of the calcination and reduction processes, can also affect directly the dispersion and the size of active species, thereby changing the catalytic activity and the product selectivity. The researchers from Beijing University of Chemical Technology prepared a nano-sized Fe-based catalyst by using a supercritical fluid technology combining with chemical precipitation, gelation, and supercritical drying methods, which led to a highly dispersed iron catalyst. With that catalyst, a CO conversion higher than 96% and the selectivity of light olefins among hydrocarbons higher than 54% were reported [Beijing University of Chemical Technology; A nano catalyst for preparing light olefins from synthesis gas and a preparation method: China, 101396662 [P]2009-04-01].
Other strategies, for instance combining Fischer-Tropsch synthesis with other reactions such as cracking reactions in a double-bed reactor were also reported [J. L. Park, Y J. Lee, K. ff Jun, J. ff Bae, N. Vi swanadham, Y H. Kim, J. Ind. Eng. Chem. 15 (2009) 847-853]. In the first reactor, the Fischer-Tropsch reaction was carried over Fe—Cu—Al catalyst under conditions of 300° C., 10 atm and GHSV=3600 h−1. Then, the effluents were passed through the second reactor at 500° C. where the cracking catalyst of ZSM-5 was packed. In that way, much C5+ products were cracked to light olefins. Thus a selectivity of light olefins among hydrocarbons was 52%, and the selectivity of the light olefins in total products was 28%.
In comparison, the technology of synthesis gas to methanol and then methanol to olefins is mature and commercialized. Therefore, there were also a lot of attempts of combining these two processes. For example, Xu et al. mixed CuO—ZnO—Al2O3 with ZSM-5 to obtain a catalyst, however, it gave mainly dimethyl ether as the product in synthesis gas conversion [M. Xu, J. H. Lunsford, D. ff Goodman, A. Bhattacharyya, Appl. Catal. A. General 149 (1997) 289; D. Mao, if Yang, J. Xia, B. Zhang, Q. Song, Q. Chen, J. Catal. 230 (2005) 140]. Erena et al. mixed multicomponent metal composites such as CuO/ZnO/Al2O3 and the like with ZSM-5 molecular sieves to catalyze the conversion of synthesis gas. However, the products were mainly gasoline [J. Erena, J. M. Arandes, J. Bilbao, A. G Gayubo, H. I. De Lasa, Chemical Engineering Science 2000, 55, 1845; J. Erena, J. M. Arandes, R. Garona, A. G Gayubo, J. Bilbao, Journal of Chemical Technology and Biotechnology 2003, 78, 161].
The present invention provides a method and catalyst for directly converting synthesis gas for production of light olefins. The selectivity of the hydrocarbon products containing 2-4 carbon atoms is up to 60%-95%, and the selectivity of light olefins including ethylene, propylene and butane is 50%-85%.